1. Field of the Invention
The invention relates to a transmission mechanism for convening a rotary movement into a translatory movement, comprising an internal coupling member which is surrounded by an external coupling member, the coupling members having mutually facing, circular-cylindrical coaxial walls with cooperating threads made of a magnetizable material, the thread of the internal coupling member having an external diameter which is smaller than an internal diameter of the thread of the external coupling member, while the transmission mechanism comprises a magnetizing member for generating a magnetic flux which permeates both coupling members.
The invention also relates to a positioning device provided with a transmission mechanism according to the invention.
The invention also relates to a lithographic device provided with a positioning device according to the invention.
2. Description of the Related Art
A transmission mechanism of the kind mentioned in the opening paragraph is known from British Patent 1 333 641. In the known transmission mechanism, the internal coupling member is a rotatable threaded spindle and the external coupling member is a nut not capable of rotation. The magnetizing member is an electric coil provided in the nut. The moment the coil is energized, a magnetic field is created which is closed through the thread of the nut and the thread of the threaded spindle. As a result, the nut and the threaded spindle enter into a position in which the threads of the nut and the threaded spindle are in opposition. When the threaded spindle is made to rotate, the threads remain in opposition under the influence of the magnetic field, the nut being displaced in axial direction relative to the threaded spindle, so that the rotary movement of the threaded spindle is converted into a translatory movement of the nut. The known transmission mechanism is used in a printer device in which a stroke mechanism is linearly displaced along a number of printing positions by the transmission mechanism against the force of a mechanical spring. The electric coil is switched off after a line has been printed, so that the magnetic field disappears and the stroke mechanism is returned to an initial position along the threaded spindle under the influence of the spring force.
A disadvantage of the known transmission mechanism is that the magnetic field in the threads of the coupling members is not homogeneous. The magnetic field has a flux density which reaches a maximum value in the turns of the threads situated near the magnetizing member, whereas the flux density is smaller in the remaining turns. As a result, the axial stiffness of the transmission mechanism and the accuracy with which the external coupling member can be positioned in axial direction relative to the internal coupling member are not optimal.